1. Field of the Invention
This invention relates to a method and apparatus for the measurement of a physical property of a fluid that is dependent upon a physical characteristic of at least one functional group and is related to the quantity of that functional group in the fluid. In one aspect, this invention relates to the measurement of the heating value of a fuel gas at-line and in real-time. In one aspect, this invention relates to a method and apparatus for measuring the heating value of a combustible gaseous fuel mixture, including functional groups and molecules, using near-infrared absorption spectroscopy. In one aspect, this invention relates to a method and apparatus for correcting the measured absorbance of an absorbing fluid to produce a true or accurate absorbance.
2. Description of Related Art
In the past, the heat energy content of a combustible fluid has been determined by burning precisely defined amounts of the combustible fluid, such as natural gas, to determine the amount of energy produced from the combustion. Other methods have determined the concentration of each whole combustible compound in the mixture, defining the energy content for each whole combustible compound, and summing them to yield the heat energy content of the entire mixture.
The heat energy content of natural gas flowing through a pipeline, which natural gas typically contains methane, ethane, propane, and higher alkane hydrocarbons, frequently fluctuates, even over relatively short periods of time. Conventional methods of measurement generally require bypass flowlines or fluid extraction to provide gas samples which are then taken to a lab and burned. The temperature of the flame is then measured. Available sensors for making these measurements are primarily calorimeters and gas chromatographs. Disadvantageously, such devices, in addition to requiring the removal of samples from pipelines, have slow response times, and have high initial and maintenance costs. It is difficult to both continuously and accurately measure the energy content of natural gas in pipelines, and the lack of any convenient method for making such continuous and accurate measurements may result in improper charges during the course of a day to the disadvantage of both buyers and sellers.
One method and apparatus for addressing the need for both continuous and accurate measurement of the heat energy content of combustible gaseous fluid mixtures is described in U.S. Pat. No. 7,248,357, which is incorporated herein in its entirety by reference. As described therein, a method and system is provided for measuring the heat energy of a combustible fluid in which radiation means direct radiation through a sample of the combustible fluid, detection means detect absorbance of at least one combustible component of the combustible fluid at a selected spectral line, where there is at least one spectral line for each combustible component to be considered in the combustible fluid, calibration means calibrate the source of the radiation, storage means store a plurality of spectra of combustible gas mixtures, thereby enabling comparison of the measured absorbance spectrum to the plurality of spectra, combination means combine at least one heat energy portion factor with the absorbance at each spectral line, and summing means sum the combinations to determine the heat energy of the combustible fluid. The system continuously acquires absorption spectra from gases in the near-infrared region. The near-infrared region of the electromagnetic spectrum is particularly useful because combustible gas components, in particular methane, ethane, propane, butane, iso-butane, and hexane produce strong absorbent spectra in this spectral range. The measurement of absorption values at several predetermined wavelengths allows reconstruction of fuel composition and heating value using specially developed mathematical algorithms. The absorbance value is calculated as
  A  =      ln    ⁡          [                        I          0                I            ]      where I0 is the light intensity measured with an optical cell filled by purging gas and I is the intensity of light measured with the cell filled with a fuel. Calibration (zeroing) of the system requires periodic flushing of the optical cell with a purging gas, such as nitrogen or air.
FIG. 1 is a schematic diagram of a conventional spectroscopic heating value sensor. As shown therein, the sensor comprises optical cell 10 having optical windows 11, 12 and input and output gas connectors 13 and 14. Periodic switching between fuel and purging gas flows is performed by valve 20. A stabilized radiation source 21 produces a radiation beam 22 that is passed through the cavity of the optical cell. The light exiting the optical cell through optical window 12 is dispersed by spectroscopic instrument 24 and directed to a near-infrared sensor array 25 measuring absorption at various wavelengths. The resulting signal is amplified by amplifier 26 and provided to data processor 27 for processing. When the cell is flushed with the zero-absorbing gas, light intensity from the source is acquired as a function of the wavelength and stored as the reference intensity I0 (λ). When fuel is flowing through the cell, light is absorbed by the fuel and a spectroscopic sensor at the other end of the cell measures the absorbance of the fuel mixture as a function of wavelength I(λ). The sensor is calibrated using absorbance spectra of fuel mixtures containing known concentrations of individual hydrocarbons at a constant pressure and temperature. During calibration, the set concentrations of known fuel mixtures are given as an input to the sensor software. Multivariate calibration techniques like Partial Components Regression (PCR) or Partial Least Squares (PLS) are utilized to form regression equations. These regression equations give individual concentrations and heating value as a function of absorbance. The heating value can be predicted directly using the regression equation or it can be calculated using the predicted concentrations.
It will be appreciated by those skilled in the art that the accuracy of the absorbance measurements depends on the stability of the reference intensity I0(λ) which, in turn, is affected, at least in part, by the stability of the radiation source including the radiation source temperature and radiation intensity, the spectroscopic sensor sensitivity and zero background drift, and the amplifier. U.S. Pat. No. 7,248,357 proposed to use special additional sensors and wavelength filters to independently monitor the radiation source intensity. Unfortunately, these factors cannot be completely eliminated, even by utilizing high stability (high-cost) hardware. Thus, there is a need to provide some means for correcting the errors in absorbance measurements arising as a result of these and other factors.